This invention relates to a power transmission of the hydro-mechanical type. In such transmissions the engine crankshaft delivers power to the vehicle wheels (or drive sprockets) through two power paths. One power path comprises a hydraulic pump-motor system; the other power path comprises a mechanical power path. Usually in the low speed range the entire engine output is directed through the hydraulic pump-motor system. As the system speed increases a progressively greater percentage of the power is directed through the mechanical path. Usually the power shift (from the hydraulic path to the mechanical path) occurs in discrete steps or gear ratios.
The present invention seeks to provide a hydro-mechanical transmission that is relatively compact and inexpensive. The hydraulic power path includes a planetary gear system wherein a planet gear carrier is held motionless or allowed to rotate according to different operational modes. In the "park" or "neutral" modes the carrier is allowed to freely rotate. In the drive modes the carrier is held motionless; operation of a hydraulic motor in one direction rotates a sun gear in one direction, producing non-orbital rotations of the planet gears in the forward drive direction. Operation of the hydraulic motor in the opposite direction causes the sun gear and non-orbiting planet gears to rotate in the reverse drive direction. A ring gear (meshed with the planet gears) constitutes the transmission output.
The mechanical power path includes a second sun gear and associated planet gears. Each planet gear has a different diameter so that each planet gear has a different rotational speed (for a given sun gear speed). Centrifugal clutch mechanisms are arranged between the planet gears in the hydraulic path and mechanical path, such that speed increase in the hydraulic power path causes the mechanical power path planet gears to be sequentially connected into the drive system.
The system utilizes a minimum number of gears and clutch mechanisms to achieve a large number of forward speed ratios. In one particular system four forward speed ratios are achieved with only two planet gear assemblies.
The transmission can be used in various vehicles or stationary installations (generator drives, pump or fan drives, winch drives, etc.). However it was particularly designed for use in tracked vehicles (e.g., military tanks). As used in tracked vehicles, two transmissions would be employed, one for each track drive sprocket. The individual transmissions would in some cases be small enough to permit the engine to be mounted in a transverse orientation (i.e., with the engine crankshaft axis extending crosswise of the vehicle longitudinal axis). A transverse orientation of the engine could somewhat reduce the overall vehicle length allocated to the power plant, thereby permitting economies in tank size, armor, and weight.